
The present invention relates to an optical encoder, and more particularly to an improvement in a structure of an optical detecting section of an optical encoder. The present invention is applicable to optical encoders of both types of a rotary type and a linear-type.
Optical encoders are widely used in detecting position and/or speed of an object performing rotational or translation motion. FIG. 1 schematically shows a general structure of an optical detecting section of a rotary-type optical encoder for detecting rotational position and/or rotational speed of a motor, by way of example.
The optical detecting section comprises a movable slit (a rotary slit in this example, a linear motion slit for a linear encoder) 1, a rotating shaft 2, a stationary slit 3, a light receiving section 4 and a light source 5, as basic elements. The light source 5 incorporates a lens or the like for making a beam parallel if necessary as well as one or more light emitting devices (for example, LEDs). Part of a beam L emitted from the light source 5 is detected by the light receiving section 4 after passing through light transmitting portions of the movable slit 1 and the stationary slit 3 in succession, and almost all of the remaining beam is obstructed by a light intercepting portion of the movable slit 1 or the stationary slit 3.
FIG. 2 generally shows a sectional structure of the movable slit 1, the stationary slit 3 and the light receiving section 4 used in the conventional optical encoder as shown in FIG. 1. As shown in FIG. 2, the movable slit 1 is a device having a function of converting the beam L emitted from the light source 5 into a plurality of beams (hereinafter referred to xe2x80x9cshifting beam groupxe2x80x9d) which are periodically distributed and shifting in accordance with a motion of an object (e.g., a rotor shaft of a motor). The movable slit 1 comprises light intercepting portions 12 and light transmitting portions 13 formed periodically on surface (one surface or both surfaces) of a transparent board 11 with a pitch of a half of a predetermined reference pitch IP.
An optical material such as a glass plate is used as the transparent board 11. The light intercepting portions 12 and the light transmitting portions 13 are formed by depositing chromium on the whole surface of the transparent board 11 and then selectively removing the deposited layer of chromium by etching. The portions at which the chromium-deposited layer is removed by the etching is formed to be the light transmitting portions 13, and the remaining portions is formed to be the light intercepting portions 12.
The structure and manufacturing of the stationary slit 3 are the same as those of the movable slit 1. Specifically, light intercepting portions 32 and light transmitting portions 33 are formed periodically on surface (one surface or both surfaces) of a transparent board 31 with a pitch of a half of the reference pitch IP which is identical with that in the movable slit 1. An optical material such as a glass plate is used as the transparent board 11, and on the surface thereof the light intercepting portions 32 are formed by using a chromium deposition layer. As in the case of the movable slit 1, the selective removing of the chromium-deposition layer by etching can be used in order to form a board regions corresponding to the light transmitting portions 33. Alternatively, the light transmitting portions 33 may be formed by machining, e.g., punching of the board 31 made of a light intercepting material.
The light receiving section 4 has light receiving elements (e.g., photodiodes) arranged on a board 41 with a pitch of a half of the reference pitch IP. When the rotating shaft 2 connected to a rotor of a motor or the like is rotated, the movable slit is rotated, so that the rotational position of the light transmitting portion 13 formed on the movable slit 1 changes. Accordingly, the scannable beam are scanned and an overlapping relationship with the light transmitting portion 33 formed on the stationary slit 3 changes periodically.
The light incident on the light receiving element (light sensing zone) 42 is converted into an electrical signal, while the light incident on a region (light non-sensing zone) where the light receiving element (light sensing zone) 42 is not provided is not converted into an electrical signal. As a result, a ratio of the amount of light incident on the light receiving element (light sensing zone) 42 of the light receiving section 4 to the amount of light contained in the scannable beam changes periodically. The periodical electrical signal thus generated is processed by a well-known processing circuit.
FIG. 2 shows a state in which a positional relationship such that the light transmitting portion 13 of the movable slit 1 agrees with the light transmitting portion 33 of the stationary slit 3.
As a technical improvement of such an encoder, there has been proposed a technique in which a condenser lens is formed at a portion overlapping with the light transmitting portion 13, 33 of the movable slit 1 or stationary slit 3 so as to prevent a phenomenon (so-called a light leakage) that a part of the light passing through the light transmitting portion 33 of the stationary slit 3 is dispersed sidewards (see Japanese Patent Laid-Open Publication No. 8-201114). Also, there is also known a structure in which the stationary slit 3 is omitted or provided between the movable slit 1 and the light source 5.
However, in the above-described conventional optical encoder, the beam from the light source is made to shift with periodical light and shade by the light intercepting/transmitting function of the movable slit, and a light detection signal is obtained in accordance with a position of the movable slit (relative to the stationary slit or the light receiving element). Therefore, a utilization efficiency of the light is poor in that at least a half of the light emitted from the light source (the hatched portion in FIG. 2) can not contribute to the signal generation at all.
Specifically, at the time when the light emitted from the light source is converted into a shifting beam, almost a half of the light amount has already been wasted, so that an efficient output signal can not be obtained. In the optical encoder proposed in the aforementioned Japanese Patent Laid-Open Publication No. 8-201114, the light dispersing sideways is collected by a condenser lens element provided so as to correspond to the light transmitting portion. However, the problem of at least a half of the light emitted from the light source being wasted is not solved.
Also, in order to form a light and shade grating with the light transmitting portion and light intercepting portion on the movable slit 1 and the stationary slit 3, a complicated and costly process such as chromium deposition, etching and machining is needed to increase the cost of the whole encoder.
A first object of the present invention is to provide an optical encoder in which a structure of an optical detecting section of a conventional optical encoder is improved to enhance a utilization efficiency of light and thus an efficient signal output is obtained. A second object of the present invention is to provide an optical encoder which does not need chromium deposition, etching, machining, etc. in forming a light and shade grating and is capable of being manufactured easily by a simple manufacturing method such as injection molding, and advantageous in terms of economy.
The present invention uses a conversion device having a lens element group arranged periodically with a predetermined pitch as means for converting a beam emitted from a light source into a plurality of shifting beams distributed periodically.
The optical encoder of the present invention includes a light source for emitting a beam; a conversion device having a plurality of lens elements arranged periodically with a predetermined pitch, for converting the beam emitted from the light source into a plurality of shifting beams periodically distributed and shifting in accordance with a motion of the object; and a light receiving section having a light sensing zone and a light non-sensing zone which are arranged to be distributed alternately and periodically along a shifting direction of the shifting beams, and disposed stationarily with respect to the shifting beams. Each of the plurality of shifting beams is converged to a width corresponding to the predetermined pitch of the arrangement of the light sensing zone and the light non-sensing zone, at the time of impinging on the light sensing zone and the light non-sensing zone.
The plurality of lens elements are provided on either or both of the light outgoing side and the light incident side of the conversion device. In a preferred embodiment, the conversion device is provided with the plurality of lens elements arranged periodically with the predetermined pitch in a pair on the light incident surface side and the light outgoing surface side. By using this structure, the beam emitted from the light source can be made parallel when going out of the conversion device.
In any mode, the conversion device having the plurality of lens elements may be a plastic-molded product.
The arrangement of the light sensing zone and the light non-sensing zone can be formed by the light receiving elements provided periodically without using a stationary slit, but the stationary slit may be used.
Unlike the conventional arrangement shown in FIG. 2, the optical encoder of the present invention eliminates a situation in which almost a half of the amount of light is wasted at the time of converting the beam emitted from the light source into the shifting beams. Therefore, a signal output with a double efficiency can be obtained in principle as compared with the conventional construction. Also, since a complicated process such as deposition, etching, and punching is not needed to form the light intercepting portion on the device for forming the shifting beams, the optical encoder of the invention is also advantageous in terms of manufacturing cost.